The old dog had been her 'constant companion' for over 12 years. [=had been with her most of the time for 12 years]
The book is a 'companion to' the television series with the same name.
The book is the 'companion' volume 'to' his previous book on jazz.
(..) - Good luck. - Thanks. Hi. I need a room for the night. Unless... you have a companion joining you later. If you have a companion, it's more. I don't have a companion. So it's $. For now. It'll stay $. (..)2014 Wild